About 300 US marines will deploy to Afghanistan's embattled southern Helmand province as part of a NATO mission to train and advise local forces - a move that the Taliban said it was a challenge that it welcomed.

The marines will head to the poppy-growing province this spring in the latest sign that foreign forces are increasingly being drawn back into the 15-year conflict.

Task Force Southwest - led by Brigadier-General Roger Turner Jr - "will train and advise key leaders within the Afghan National Army 215th Corps and the 505th Zone National Police", a statement published late Friday by the US Marine Corps said.

"The Marine Corps has an operational history in Afghanistan, particularly in Helmand province," the statement noted, adding the troops "will assist in preserving gains made together with the Afghans".

The soldiers will train and advise Afghan security forces pushing back against Taliban fighters who have taken most of Helmand after the end of NATO's combat mission and withdrawal of most US forces from the province in 2014.

Almost all of Helmand's districts, except for the provincial capital, Lashkar Gah, are either heavily embattled or fully controlled by the Taliban.

"We really need air support in Helmand. I hope they [US] support our air force, since we don't have enough air power in Helmand," Rasul Zazai, a spokesman for the Afghan army's 215 Corps stationed in Helmand, told AFP news agency.

Afghanistan war map who controls what [Al Jazeera]

In response to the marines' announcement, the Taliban told Al Jazeera on Saturday that Helmand is under "strong" control and the group is "happy to fight against them and maintain their stronghold in Helmand".

"This statement is a response beforehand to our upcoming spring offensive. This is just to boost the morale of the Afghan forces who were brutally beaten by our fighters in Helmand," Zabihullah Mujahid, a Taliban spokesman, told Al Jazeera.

"Let them come and fight with us, so we can show them how strong we are and how they they will once again fail miserably in the fight to take back Helmand."

Helmand is strategically important for the Taliban, as it is the main source of the country's opium output, worth an estimated $4bn a year, much of which funds the war.

Provincial officials say the Taliban now controls 85 percent of the province, while only a year ago the government controlled 80 percent.

About 30,000 people were displaced in Helmand alone, mostly fleeing to the provincial capital, with the city still practically besieged. Roads from neighbouring districts are heavily mined by the Taliban.

NATO officially ended its combat mission in December 2014, but US forces were granted greater powers in June last year to strike at the Taliban fighters as President Barack Obama vowed a more aggressive campaign.

The United States still has about 8,400 troops in the country.

In early December, General John Nicholson, the chief US and NATO commander in Afghanistan, said the government directly controls about 64 percent of the population of 30 million, down slightly from 68 percent earlier in 2016.